AAL2 is a packet based ATM standard (ITU-T I.366) that allows multiple users to share the same Virtual Path Identification/Virtual Channel Information (VPI/VCI). FIG. 1a elaborates on the AAL2 approach in more detail. Within an AAL2 framework, AAL2 packets (such as the packet formed by header 101a and payload 102a and the packet formed by header 101b and payload 102b) are transported over an ATM network.
As ATM cells are used to transport information over an ATM network, the AAL2 packets observed in FIG. 1a are “broken down” into separate pieces of information that are each carried by individual ATM cells (which are not shown in FIG. 1a for simplicity). Upon their reception, the payload of the ATM cells are pieced back together to form the flow of packets.
Frequently, neighboring packets are associated with different connections. For example, the packet formed by header 101a and payload 102a may be destined for a first user while the packet formed by header 101b and payload 102b may be destined for a second user. Thus, within each header 101a, 101b, is a Connection Identification (CID) label that informs a receiving node as to which connection the corresponding payload 102a, 102b belongs.
Under the current ITU-T I.366 specification, 6 bits are reserved within the CID which allows for 248 different connections. As such, up to 248 different AAL2 connections can be carried by a single VPI/VCI ATM connection. Note that information within different AAL2 packets may be carried by the same ATM cell. That is, the payload of a single ATM cell may contain information at (and on either side of) the transition X from the first packet to the second packet seen in FIG. 1a. As such, a pointer may also be included in the packet header to signify where a packet starts and where a packet ends.
FIG. 1b shows a model 103 for an AAL2 switch. Ingress ATM cells are received on a plurality of ingress lines (such as ingress line 108). A line, such as an ingress line, may be any line that carries ATM cells (such as any OC-n line or STS-n line). The ATM cell traffic from the ingress lines are collected on the ingress portion 105a, 106a, 107a of one or more line cards.
The ingress portion 105a, 106a, 107a of each of the line cards reconstructs AAL2 packets from the payloads of the received ATM cells. The AAL2 packets are then forwarded to an AAL2 switch core 104 that switches each ingress AAL2 packet, based upon its respective CID, to the appropriate egress line card portion 105b, 106b, 107b. The AAL2 switch core 104 can change the CID label to an appropriate egress value for transmission from the switch 103. Note that, frequently, a single line card has both an ingress portion and an egress portion.
Thus, for example, ingress portion 105a and egress portion 105b may be viewed as separate portions of a first line card; ingress portion 106a and egress portion 106b may be viewed as separate portions of a second line card; and ingress portion 107a and egress portion 107b may be viewed as separate portions of a third line card. Upon receipt of an AAL2 packet at an egress portion, the AAL2 packet may be broken down into pieces that are carried by different ATM cells which are subsequently transmitted along the appropriate egress line (such as egress line 109).
A problem, however, is the loss of economies of scale when implementing an AAL2 switch with an AAL2 switch core 104. That is, AAL2 is a specific type of networking approach. As other types of networking approaches are in common usage (e.g., pure ATM, AAL3/4, AAL5, Packets Over SONET (PoS), etc.), a switch having an AAL2 switch core 104 can not efficiently switch the traffic associated with these other types of networking approaches.